I'm reading a book called Modern Marine Weather, I'm only a few pages in, and reading an explanation on wind speed in relation to actual wind pressure, set off a light bulb for me!
An excerpt to set the scene!
"wind force does not increase in direct proportion to wind speed"
"but rather in proportion to the square of the wind speed"
Example:
a 10 knot wind has twice the force of a 7 knot wind,
(since 10x10 is about twice as large 7x7)
a wind speed of 21 knots does not push the boat 3-times harder than a 7 knot breeze; it pushes, or puts 9 times the force on the sails and rig!
if you refer to my rather mundane looking post on Storm Clouds I experienced sailing back to Port Hacking a week or two back, from Lake Macquarrie; I was at the time the picture was taken in about 5 knots of wind, and motoring with no sails, as I was heading down wind. In the shake of a lambs tail, soon after that picture was taken, I saw the wind gauge hitting 46 knots, coming directly off the bow, IE a Southerly. I didn't take any pictures, I was, ahem busy! But using the rule from above;
5x5 = 25
46x46 = 2116
2116 divided into 25 = 84.64
so I experienced a rapid increase in wind pressure 80 odd times what I was basking in 5 minutes earlier.
Im glad I had stowed there sails away earlier, due to them flapping and flogging in the downwind light conditions with a rolling sea state, my good luck I guess?
This dovetails nicely into Bundeena's post on sail trim, it's not hard for me to understand how sails can quickly get out of trim in what seems like small increases in wind speed! When you understand the actually pressures in play!
sorry for the long winded post, pun intended!
Richard
Select to expand quoteStockie said..
I'm reading a book called Modern Marine Weather, I'm only a few pages in, and reading an explanation on wind speed in relation to actual wind pressure, set off a light bulb for me!
An excerpt to set the scene!
"wind force does not increase in direct proportion to wind speed"
"but rather in proportion to the square of the wind speed"
Example:
a 10 knot wind has twice the force of a 7 knot wind,
(since 10x10 is about twice as large 7x7)
a wind speed of 21 knots does not push the boat 3-times harder than a 7 knot breeze; it pushes, or puts 9 times the force on the sails and rig!
if you refer to my rather mundane looking post on Storm Clouds I experienced sailing back to Port Hacking a week or two back, from Lake Macquarrie; I was at the time the picture was taken in about 5 knots of wind, and motoring with no sails, as I was heading down wind. In the shake of a lambs tail, soon after that picture was taken, I saw the wind gauge hitting 46 knots, coming directly off the bow, IE a Southerly. I didn't take any pictures, I was, ahem busy! But using the rule from above;
5x5 = 25
46x46 = 2116
2116 divided into 25 = 84.64
so I experienced a rapid increase in wind pressure 80 odd times what I was basking in 5 minutes earlier.
Im glad I had stowed there sails away earlier, due to them flapping and flogging in the downwind light conditions with a rolling sea state, my good luck I guess?
This dovetails nicely into Bundeena's post on sail trim, it's not hard for me to understand how sails can quickly get out of trim in what seems like small increases in wind speed! When you understand the actually pressures in play!
sorry for the long winded post, pun intended!
Richard
Wow!
A lightbulb moment for me.
Another question would be if the density of the air and temperature makes a difference to the effect on sails.
Select to expand quotesaltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.
Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue
Temperature can make a difference with wind shear and that does affect your sail trim.
Take up Kitesurfing and you'll realise there's a vast difference between 15 knts and 20 knts. Puts in to practice what put you to sleep in Maths class at school.
Force (N) = mass (kg) ? acceleration (m/s?)
One of many things to remember when reading weather maps / isobars is that the lines drawn represent pressure and not exactly wind direction. Allow about 20 degrees Inward for a low and same outward for a high, off the pressure lines themselves. Winds travel in a spiral in or out of a system. Highs will generally dominate and push away a low and weather will do whatever it likes 
Select to expand quoteCrusoe said..
Take up Kitesurfing and you'll realise there's a vast difference between 15 knts and 20 knts. Puts in to practice what put you to sleep in Maths class at school.
Force (N) = mass (kg) ? acceleration (m/s?)
Speaking of which, general rule of thumb is that wind is about 25% stronger 10 metres up compared to eye level. One of the first things we learn in sky sports.
and weather will do whatever it likes
Went out on Friday.
Predicted weather, 4 to 12 knots SSE turning to E
Reality, 0 knots raising to 10 knots from the N turning to 15 from the west.
Of course, I planned a sailing route according to forecast!
Gary
All sailors should understand the Buys Ballot law principles. My dad taught me it when I was a kid and to this day I subconsciously apply Buys Ballot and compare to weather maps even if I'm not on a boat.
I recall a friend talking about dinghy racers coming down to Tasmania from Sydney and getting into trouble with their rig tunings because of the greater force in the wind here at any given wind speed. Here's an interesting explanation of the relationship between temperature, humidity, air density and the resulting force. My take away is that a drier colder airstream down here in Tasmania (given the same wind speed) might pack upwards of 5-10% more punch compared to the hotter, more humid parts of the country. What's your reading of this?
weather.mailasail.com/Franks-Weather/Weight-Of-Wind
Select to expand quotesaltiest1 said..Crusoe said..
Take up Kitesurfing and you'll realise there's a vast difference between 15 knts and 20 knts. Puts in to practice what put you to sleep in Maths class at school.
Force (N) = mass (kg) ? acceleration (m/s?)
Speaking of which, general rule of thumb is that wind is about 25% stronger 10 metres up compared to eye level. One of the first things we learn in sky sports.
Yeah, hence the need for sail twist as the angle of attack is different at the top of the sails.
Select to expand quotelydia said..saltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.
Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue
Temperature can make a difference with wind shear and that does affect your sail trim.
I get that the air mass is far greater in altitude, but not how that affects gusts on the ground, can you explain how that works? I'd always assumed that the wonky westerlies down here were due to the turbulence caused across the mountains and hills to the west.
Select to expand quote2bish said..lydia said..saltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.
Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue
Temperature can make a difference with wind shear and that does affect your sail trim.
I get that the air mass is far greater in altitude, but not how that affects gusts on the ground, can you explain how that works? I'd always assumed that the wonky westerlies down here were due to the turbulence caused across the mountains and hills to the west.
The breeze you sail in does not simply flow parallel to the water surface but flow downwards and from the boundary layer.
So in a sea breeze the gust so to speak will fall from 500m while in the channel, 3000 metres.
Greater hieght and greater accelerations
So the channel the breeze is coming down from say 3000m and then on top you have the disturbance caused by the hills which cause further acceleration and deacceleration of that air flow.
The stretch from Gordon to Oyster Cove in a westerly is a good example.
It is like the old Tasmanian cruising saying, never anchor behind the big hill as it speeds up the breeze.
Select to expand quotelydia said..2bish said..lydia said..saltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.
Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue
Temperature can make a difference with wind shear and that does affect your sail trim.
I get that the air mass is far greater in altitude, but not how that affects gusts on the ground, can you explain how that works? I'd always assumed that the wonky westerlies down here were due to the turbulence caused across the mountains and hills to the west.
The breeze you sail in does not simply flow parallel to the water surface but flow downwards and from the boundary layer.
So in a sea breeze the gust so to speak will fall from 500m while in the channel, 3000 metres.
Greater hieght and greater accelerations
So the channel the breeze is coming down from say 3000m and then on top you have the disturbance caused by the hills which cause further acceleration and deacceleration of that air flow.
The stretch from Gordon to Oyster Cove in a westerly is a good example.
It is like the old Tasmanian cruising saying, never anchor behind the big hill as it speeds up the breeze.
Another to consider is that for every metre high obstacle, 10 metres horizontal is needed to attain clean air again.
When sailing you want accelarating air.
Great example is finishing a Hobart from the Iron Pot to the finish in a sea breeze.
The eastern shore after the Pot, stay close to the shore and sail into every bay.
The breeze comping over the land accelerates as it hits the water edge. No friction.
So more breeze on the windward shore rather than the centre of the river.
Same applies to the northern NSW coast at night with cold air drainage.
Stay almost exactly 1 mile of the beach at all time for best pressure.
any closer and the air stream is not all connected and any further out the strength dissipates.
As the shoreline falls away sail into the bay and then do the opposite.
You will get more pressure with bigger hills say around Coffs and get less further north where the land is not so high.
Of course you need to be much closer on the headlands.
Gets tricky at night around some of the headlands especially Double Island Point for instance but I have the theory around that one where you need to stand off more.
